I also really hope my suggestions are just bullshit. Especially since those have the worst implications. But I would still be watching out for similar events. Just in case :)

I am not sure if this is a joke. Most commentators seem to perceive it this way. But just in case I will answer seriously.

This was done by a human:

• your kid (you said you live alone, so not this one)
• your guests
• someone uninvited (but it makes no sense for them to leave this)
• you in a way you cannot remember.

I will expand on that last one since no one has done it: Do you have an issue with substance abuse? Do you have issues with your memory? Do you have any psychiatric issues? Your other personality could to this if you have a dissociative identity disorder. Check symptoms and pay attention on similar events. If this keeps happening go see a doctor.